This plugin can decorate all pages of Jenkins with your customized message, as shown below:
View Example
Clone this repo on your disk, navigate into folder broadcaster-page-decorator-plugin and run command mvn verify, when it completed, upload target/broadcaster-page-decorator.hpi to the update center of Jenkins and install it.
Also you can download broadcaster-page-decorator.hpi from release page, then upload it to the update center of Jenkins.
Go to manage > configure > Broadcaster Page Decorator Plugin, just pust what you want to broadcast in the Message textarea, for example (the font color and background can be modified if needed):
Configuration Example